Studies document ease of ecosystem disruption

03/25/1999

CORVALLIS, Ore. - Two new studies suggest that ecosystems can be far more vulnerable than often assumed, subject to disruption by fairly small environmental changes or loss of "minor" species not traditionally thought to be important - and in considerable peril from global change.

Both research efforts were done by ecologists from the Department of Zoology at Oregon State University on the marine rocky intertidal shores of the Oregon coast.

One report, to be published Friday in the journal Science, indicates that some ecological impacts of global warming might be abrupt, significant, and generally underestimated - not just a slow shift of species from one region to another. It found that small changes in ocean temperature could affect important or "keystone" species and trigger large, relatively rapid changes in intertidal ecology.

The other report, published today in the journal Nature, suggests that measures to protect ecosystem health and function must consider not only those keystone species known to play dominant roles, but also many less prominent species which, at various times, may actually be highly important.

Together, the research findings imply that the function of complex ecosystems is both difficult to predict and sometimes surprisingly easy to disrupt, especially with the advent of human-related stresses such as over-exploitation, increased species extinctions and climate change.

"As we consider the impacts of global warming, many people assume the effects will be gradual, a shift to new regions by various plant and animal species," said Eric Sanford, an OSU ecologist. "But this study shows that if you have an important species which is highly sensitive to temperature, then the effects of small temperature changes on an ecosystem can be amplified by species interactions."

In his research, Sanford looked at the ochre sea star, which feeds on the California mussel and in the Pacific Northwest intertidal ecosystem holds this dominant competitor in check. But Sanford found that the rates of predation by this sea star are very sensitive to small changes in water temperature associated with episodes of wind-driven upwelling. It's one of the first experiments of its type to document a direct link between small temperature changes and the effects of a keystone species.

"Different species interact constantly through such mechanisms as predation, competition and disease," Sanford said. "Because of that, an impact on a keystone species may cascade through the community and cause a relatively quick, system-wide impact."

The new study reported in Nature, on the other hand, looked not so much at the keystone species of an ecosystem but the comparatively minor species that often get ignored.

Its findings challenge some conventional wisdom about ecological protection.

"Our tendency is to focus research and management efforts on species that have consistent, dramatic impacts on an ecosystem," said Eric Berlow, a recent OSU doctoral graduate now doing research at the University of California, Berkeley.

Underlying that approach is the assumption that by maintaining populations of the species believed to be the most important, the larger ecosystem will be protected and the lesser species can be ignored - or even exploited - because they are of little ecological significance.

Berlow's study challenges that thesis, and in the process makes a strong case for the management and protection of whole ecosystems, not individual species.

In his experiment, Berlow examined the dog whelk, a marine snail that acts as a predator on mussels and barnacles. Unlike many terrestrial predators, such as wolves and mountain lions, it is easy to conduct experiments with rocky intertidal organisms, Berlow said. In this case he manipulated whelk abundance to compare the impacts of strong versus weak species effects.

While strong predation on mussels had a consistent effect under all the conditions examined, the effects of weak predation were highly variable. In individual situations, weak predation actually had important effects on mussel abundance, but these effects varied from place to place and time to time.

"The effects were visually dramatic," Berlow said. "Some weak predation plots were dominated by mussels while others had none. So even though the effects of weak predation by whelks 'averaged out' to about zero, it would be a serious mistake to treat the effects as minor or insignificant.

"Sometimes what we consider to be noise is in fact an important part of the signal," Berlow said. "We just weren't listening right, we haven't always measured the right things."

In a management context, Berlow said, the experiment makes a case for managing whole ecosystems, since ecosystem health is likely to depend on the interactions of many species, the abundant ones as well as the rare ones, the keystones and the "not-so-keystones."

And in efforts to better understand ecosystem function, he said, researchers will have to begin paying far closer attention to seemingly unimportant plant or animal species.